The present invention relates to a method of forming a thin film. More particularly, the present invention relates to a method of forming a thin film using an atomic layer deposition (ALD) method.
In general, a thin film is used as a dielectric of a semiconductor device, as a transparent conductor of a liquid-crystal display, and as a protective layer of an electroluminescent thin film display. Such a thin film may be formed by a sol-gel method, a sputtering method, an electroplating method, an evaporation method, a chemical vapor deposition (CVD) method, or an ALD method.
Among these methods, it is possible to obtain a better step coverage by an ALD method than by a CVD method. Furthermore, it is possible to perform low temperature processing by the ALD method. Thus, an ALD method is more preferable in some circumstances.
In an ALD method, the thin film is formed by decomposing a reactant not by pyrolysis, but by a chemical exchange caused by the periodic supply of respective reactants. A method of forming an aluminum oxide film that can be used as a dielectric film of a semiconductor device using a conventional ALD method will be described in detail below.
FIG. 1 is a flowchart of the process of forming an aluminum oxide film using a conventional ALD method. FIGS. 2A through 2D describe the reaction mechanism during the formation of the aluminum oxide film by the method of FIG. 1.
In particular, a first reactant A, e.g., trimethylaluminum (Al(CH3)3, xe2x80x9cTMAxe2x80x9d), composed of aluminum a1 and a methyl ligand a2, is injected into a reaction chamber (not shown), containing a silicon substrate (step 1). The reaction chamber is then purged of any physisorbed first reactant A by injecting an inert gas (step 3) into the chamber. Thus, only the first reactant A that is chemisorbed into a substrate S remains bonded to the substrate S as shown in FIG. 2A.
A second reactant B, e.g., water vapor, consisting of oxygen b1 and a hydrogen radical b2, is then injected into the reaction chamber, which contains the substrate S onto which the first reactant A is chemisorbed (step 5). In this way, the second reactant B is chemisorbed onto the first reactant A, specifically via the oxygen b1, as shown in FIG. 2B.
The hydrogen radical b2 of the chemisorbed second reactant B then moves to the methyl ligand a2 of the first reactant A causing the methyl ligand a2 to separate from the aluminum a1, as shown in FIG. 2C. Then, as shown below in chemical formula (1) and in FIG. 2D, the hydrogen radical b2 of the second reactant B reacts with the separated methyl ligand a2 of the first reactant A and forms a volatile vapor phase material D formed of CH4. An aluminum oxide film C is then formed on the substrate S by the reaction between the aluminum a1, of the first reactant A and the oxygen b1, of the second reactant B.
2Al(CH3)3+3H2Oxe2x86x92Al2O3+6CH4xe2x80x83xe2x80x83(1)
The volatile vapor phase material D formed of CH4 and the un-reacted vapor are then removed by purging the reaction chamber with an inert gas (step 7). Finally, it is necessary to determine whether the aluminum oxide film is formed to an appropriate thickness (step 9). If so, the process ends; if not, then steps 1 through 7 are cyclically repeated as necessary.
However, in a conventional ALD method, since the methyl ligand a2 is removed by the movement of the hydrogen radical b2, a sub-reaction occurs producing an OH radical that remains according to the movement of the hydrogen radical b2, as described in chemical formula (2).
Al(CH3)3+3H2Oxe2x86x92Al(OH)3+3CH4xe2x80x83xe2x80x83(2)
When this sub-reaction occurs, undesired impurities such as Al(OH)3 are included in the aluminum oxide film C. And when impurities such as Al(OH)3 are included in the film C, it is not possible to obtain the desired thin film characteristics. In particular, when an aluminum oxide film including Al(OH)3 is used as a dielectric film of a semiconductor device, the portion of the aluminum oxide film that includes Al(OH)3 operates as a trap site for electrons or a current leakage site, thus deteriorating the characteristics of the entire dielectric film.
It is an object of the present invention to provide a method of forming a high purity thin film by suppressing the formation of undesired impurities when an atomic layer deposition (ALD) method is used.
To achieve the above object, a method is provided of forming a thin film over a substrate using atomic layer deposition (ALD). The method includes placing the substrate into a reaction chamber; injecting a first reactant into the reaction chamber so that a portion of the first reactant is chemisorbed onto the substrate, the first reactant including a thin film atom and a ligand; purging the reaction chamber with a first inert gas to remove any of the first reactant that is only physisorbed onto the substrate; and injecting a second reactant into the reaction chamber to form the thin film in atomic layers by a chemical reaction between the thin film atom and the second reactant, and to remove the ligand without generating by-products. In this method, the binding energy of the second reactant with respect to the thin film atom is larger than the binding energy of the ligand with respect to the thin film atom.
The first reactant preferably comprises Al(CH3)3 and the second reactant comprises an activated oxidizing agent. The activated oxidizing agent preferably comprises a material selected from the group consisting of O3, O2 plasma, and N2O plasma.
The method may further include purging the reaction chamber with a second inert gas, after injecting the second reactant, to remove any of the second reactant that is physisorbed onto the substrate. The first and second inert gases may be the same gas.
The injecting of the first reactant into the reaction chamber, the purging of the reaction chamber with a first inert gas, the injecting of the second reactant into the reaction chamber, and the purging of the reaction chamber with a second inert gas, may be repeated a plurality of times.
According to the present invention, the ligand of the first reactant is separated by the difference in binding energy without the movement of a radical from the second reactant to the first reactant. A volatile vapor phase material is formed by the combination of ligands and the vapor phase material is purged. Accordingly, since it is possible to reduce the impurities generated in the thin film by a sub-reaction without the movement of the radical, it is possible to obtain a high purity thin film.
An alternate method of forming a thin film using an ALD method is also provided. This alternate method includes placing a substrate into a reaction chamber; injecting a first reactant into the reaction chamber so that a portion of the first reactant is chemically adsorbed onto the substrate; purging the reaction chamber with a first inert gas to remove any first reactant that is only physisorbed over the substrate; injecting a second reactant into the reaction chamber to chemically exchange a first portion of the chemisorbed first reactant and to form a metal-oxygen atomic layer film; purging the reaction chamber with a second inert gas to remove any of the second reactant that is physisorbed over the substrate; and injecting a third reactant into the reaction chamber to form a metal oxide film in units of atomic layers by chemically exchanging a second portion of the chemisorbed first reactant. In this method, the second reactant does not contain a hydroxide. The injecting of the third reactant into the reaction chamber operates to prevent the generation of hydroxide.
The first reactant is preferably a metal reactant, the second reactant is preferably N2O, O2, O3, or CO2, and the third reactant is preferably an oxidizing gas. The reaction chamber is preferably maintained to be between 100 and 400xc2x0 C. from the injecting the first reactant to the injecting the third reactant. The metal oxide film preferably comprises a material selected from the group consisting of an Al2O3 film, a TiO2 film, a ZrO2 film, an HfO2 film, a Ta2O5 film, an Nb2O5 film, a CeO2 film, a Y2O3 film, an SiO2 film, an In2O3 film, an RuO2 film, an IrO2 film, an SrTiO3 film, a PbTiO3 film, an SrRuO3 film, a CaRuO3 film, a (Ba,Sr)TiO3 film, a Pb(Zr,Ti)O3 film, a (Pb,La)(Zr,Ti)O3 film, an (Sr,Ca)RuO3 film, a (Ba,Sr)RuO3 film, an In2O3(ITO) film doped with Sn, and an I2O3 film doped with Zr.
A dangling bond on a surface of the substrate is preferably terminated by injecting an oxidizing gas into the reaction chamber before injecting the first reactant. In this case, the substrate is a silicon substrate.
The method may further include purging the reaction chamber with a third inert gas after injecting the third reactant into the reaction chamber, to remove any of the third reactant physisorbed over the substrate. The first, second, and third inert gases are preferably the same.
The injecting of the first reactant into the reaction chamber, the purging of the reaction chamber with the first inert gas, the injecting of the second reactant into the reaction chamber, the purging of the reaction chamber with the second inert gas, the injecting of the third reactant into the reaction chamber, and the purging of the reaction chamber with the third inert gas, may be repeated a plurality of times.
The method may further include injecting a fourth reactant into the reaction chamber, after purging of the reaction chamber with the third inert gas, to remove any impurities and to improve the stoichiometry of the metal oxide film. The fourth reactant is preferably ozone gas.
Yet another method is provided of forming a thin film using an ALD method. This method includes loading a substrate into a reaction chamber; injecting a first reactant into the reaction chamber so that a portion of the first reactant is chemically adsorbed into the substrate; purging the reaction chamber with a first inert gas to remove any of the first reactant that is only physisorbed over the substrate; injecting a second reactant into the reaction chamber to form the thin film in units of atomic layers by chemically exchanging the first reactant to further contribute to the formation of the second reactant; purging the reaction chamber with a second inert gas to remove any of the second reactant that is physisorbed over the substrate; and injecting a third reactant into the reaction chamber to remove impurities and improving the stoichiometry of the thin film.
The first reactant preferably comprises a metal reactant and the second and third reactants comprise oxidizing gases. The thin film preferably comprises a metal oxide film formed of a single atomic oxide or a composite oxide. The single atomic oxide preferably comprises a material selected from the group consisting of Al2O3, TiO2, Ta2O5, ZrO2, HfO2, Nb2O5, CeO2, Y2O3, SiO2, In2O3, RuO2, and IrO2. The composite oxide preferably comprises a material selected from the group consisting of SrTiO3, PbTiO3, SrRuO3, CaRuO3, (Ba,Sr)TiO3, Pb(Zr,Ti)O3, (Pb,La)(Zr,Ti)O3, (Sr,Ca)RuO3, In2O3 doped with Sn, In2O3 doped with Fe, and In2O3 doped with Zr. The first reactant is preferably a metal reactant and the second and third reactants are preferably nitriding gases. The thin film preferably comprises a metal nitride film formed of a single atomic nitride or a composite nitride. The single atomic nitride preferably comprises a material selected from the group consisting of SiN, NbN, ZrN, TiN, TaN, Ya3N5, AlN, GaN, WN, and BN. The composite nitride preferably comprises a material selected from the group consisting of WBN, WSiN, TiSiN, TaSiN, AlSiN, and AlTiN.
The method may further include purging the reaction chamber with a third inert gas, after injecting the third reactant, to remove any of the third reactant that is physisorbed over the substrate. The first, second, and third inert gases are preferably the same gas.
The injecting of the first reactant into the reaction chamber, the purging of the reaction chamber with the first purge gas, the injecting of the second reactant into the reaction chamber, the purging of the reaction chamber with the second purge gas, the injecting of the third reactant into the reaction chamber, the purging of the reaction chamber with the third purge gas, may be repeated a plurality of times.
A dangling bond on a surface of the substrate may be terminated by injecting oxidizing gas or nitriding gas into the reaction chamber before injecting the first reactant. In this case, the substrate is a silicon substrate.
The temperature of the reaction chamber is preferably maintained to be between 100 and 400xc2x0 C. from the injecting of the first reactant to the injecting of the third reactant.
According to these methods of forming the atomic layer thin film of the present invention, it is possible to prevent or suppress the formation of an undesired by-product such as hydroxide, to thus obtain a high purity thin film.